#include <iostream>
#include <cmath>
using namespace std;

long double area_of_square(long double side = 0){
	return side * side;
}


long double area_of_circle(long double radius = 0){
	return radius * radius * M_PI;
}


long double area_of_triangle(long double side1 = 0,long double side2 = 0,long double side3 = 0){
	long double p = (side1+side2+side3)/2;
	return sqrtl(p*(p-side1)*(p-side2)*(p-side3));
}


int main(){
	long double a;
	cout<<"To calculate the area of a square, please input the length of one side:";
	cin>>a;
	cout<<"area_of_square("<<a<<"):"<<area_of_square(a)<<endl;

	long double b;
	cout<<"To calculate the area of a circle, please input the length of one radius:";
	cin>>b;
	cout<<"area_of_circle("<<b<<"):"<<area_of_circle(b)<<endl;

	long double c,d,e;
	cout<<"To calculate the area of a triangle, please input the length of three side(separated with space):";
	cin>>c>>d>>e;
	cout<<"area_of_triangle("<<c<<","<<d<<","<<e<<"):"<<area_of_triangle(c,d,e)<<endl;
}
